0. A. Derby — Gold-hearing Lode of Passagem. 187 



the most part even in these cases the microscope reveals only 

 a micaceous afferesrate which treatment with the acid shows to 

 be heavily charged with calcite. A single inclusion only (3 in 

 fig. 2) amongst the score or more examined retains the feldspar 

 substance in a sufficiently fresh condition to permit of a satis- 

 factory microscopic and chemical examination. The micro- 

 scopic sections show in great perfection the characteristic 

 twinning bands of a plagioclase feldspar with an extinction 

 angle indicative of a nearly normal oligoclase, and an approxi- 

 mate analysis confirms this reference, as does also the external 



Fig. 2. 



form, since with careful work many of these inclusions can be 

 completely detached from the matrix in ideally sharp-cut crys- 

 tals. 



These inclusions are, therefore, original feldspars that have 

 become sericiticized by some agency that has reached them 

 through an apparently impervious involucre of dense vein 

 quartz. In their immediate vicinity the quartz shows fissures 

 large and small, filled with tourmaline and pyrites which have 

 evidently been introduced by solutions, or gases, carrying boron, 

 sulphur and arsenic along with iron and, rarely, gold, silver, 

 copper, bismuth, lead and probably tellurium. A relation 

 between the two phenomena of the sericitization of the feld- 

 spar and the mineralization of the lode, with boron and 

 sulphur compounds, is at once suggested and fortunately defi- 

 nite evidence on this point is at hand. 



